1. Field of the Invention
The present invention relates to a mask for screen printing and methods for manufacturing the same.
2. Description of the Related Art
Heating wires are typically disposed on the rear glass (rear window or rear windshield) of an automobile to prevent or clear fogging (condensation). Specifically, two bus bars extend in the vertical direction and are disposed on the left and right sides of the rear glass. A plurality of heating wires extends in the transverse direction (horizontal direction) between the two bus bars. The pair of bus bars and the heating wires are typically applied to the rear glass by screen printing.
In addition, a border (black enamel coating or “frit”) formed by a black ceramic ink is typically disposed along the peripheral edge of the front glass (front window or front windshield) of the automobile. A main portion of the border is filled in with the ceramic ink, i.e. it is continuous and opaque, while a discontinuous pattern, e.g., made up of a plurality of dots (i.e. a dot matrix), is disposed along the inner edge of the continuous opaque band. The border is also typically applied to the front glass by screen printing.
Screen printing generally includes the following features. A printing mask is prepared by forming a liquid-impermeable resin layer (stencil or blocking stencil) on a mesh material (main mesh), such as, e.g., a woven fabric having minute openings or a metal screen. One or more through-holes (openings or open spaces) is (are) formed in the resin layer (main resin layer) in accordance with a predetermined printing pattern. The stencil with the through-hole(s) appears as a negative image of the pattern to be printed on a substrate Ink is then placed on the mesh, and a squeegee is moved (slid) across the mesh while pressing the ink and the mesh. As a result, ink passes through the through-hole(s) of the resin layer, and the predetermined pattern is printed on the substrate (material to be printed). In doing so, the ink is printed on the substrate substantially uniformly (i.e. the ink is applied with an identical thickness across the pattern), and the thickness of the ink basically corresponds to the thickness of the mesh (also referred to as the “mesh thickness”).
The bus bars disposed on the rear glass of automobiles are ordinarily designed to have one or more parts (portion(s) or area(s)) that is (are) thicker than the remaining portions of the bus bars. Specifically, the greater part of the bus bars is formed to be very thin, e.g., to minimize manufacturing costs, while one or more parts of the bus bars is (are) formed thicker than other portions, e.g., in order to moderate (reduce) the electrical resistance of the bus bars, or to provide a contact pad for an antenna terminal therein.
With respect to the border provided along the peripheral edge of the front glass of automobiles, the continuous portion of the border has to be printed thickly, since a thin border might be light permeable (i.e. the border should be opaque for aesthetic and technical reasons). Furthermore, the continuous portion (more accurately, the greater part thereof, excluding the peripheral edge) can be printed roughly (i.e. with a rough surface) without any problems. On the other hand, the dot matrix printed along the inner edge of the border has to be printed finely/precisely, since both printed and non-printed sites are intricately present within the dot matrix. That is, the dots of the dot matrix should have precise or sharp circular shapes/edges, e.g., to provide an appealing aesthetic appearance.
In order to increase the thickness of certain portions (i.e. to form thick sections) of the resulting printed pattern, conventional screen printing involves the following steps. First, screen printing is performed once, across the entire design, at the same thickness. Subsequently, only the portion(s) at which a thick section must be formed is (are) screen-printed again a second time. However, this approach is inefficient, since the screen printing operation has to be carried out twice. Furthermore, two printing masks are required, which entails higher costs. Moreover, special alignment measures/devices may be necessary to ensure that the thick portion(s) is (are) accurately printed at the intended location(s) within the think portion(s) of the printed design/structure.
As was mentioned above, the thickness of the printed ink applied by screen printing is determined by the thickness of the mesh (mesh thickness). Thus, if a relatively thick mesh is used, the amount of printing liquid (ink) that is held in the mesh (i.e. in the through-holes) is relatively large. Accordingly, the printing agent can be applied relatively thickly onto the substrate (material to be printed) using a relatively thick mesh.
Coarse or rough printing results when a mesh having a large mesh thickness is used, whereas fine (precise) printing results when a mesh having a small (thin) mesh thickness is used.
It is noted that U.S. Pat. No. 4,958,560 and its counterpart Japanese Patent Application Publication No. S63-233838 disclose a printing technique for increasing the thickness of certain portions of the printed design. This technique involves providing a local support between the screen material (main mesh) and the substrate. The local support is formed as a patch comprised of a plurality of spots, or comprised of another screen material (mesh).
However, the specific manner in which the local support (in particular, the spots) is provided in the main mesh is unclear. Furthermore, if the patch is a screen material and is bonded to the main screen (main mesh) using an adhesive, some of the adhesive may clog the openings and may thus create a printing problem if the printing agent (ink) can no longer pass through the screen.